1. Field of the Invention
This invention relates to the formation of contacts between metals and semiconductor materials, and, more particularly, to the achievement of ohmic contacts which are substantially rectification-free.
2. Description of the Related Art
Contacts between metallic and semiconductor materials occur in a very large number of integrated circuit and other semiconductor devices. Such junctions typically exhibit a rectifying effect, and in many cases, it would be desirable to eliminate this rectification. Theoretically, a metallic element or compound should make a non-rectifying (ohmic) contact with a semiconductor if the two materials form an ideal Schottky junction. This type of junction was first postulated by W. Schottky in an article published in Physik Zeits, 41, 570 (1940). A non-rectifying Schottky junction was the predicted result of forming (a) a junction between a metallic element or compound and an n-type or intrinsic semiconductor material when the work function of the metallic material was less than that of the semiconductor material, or (b) a junction between a metallic material and a p-type or intrinsic semiconductor material when the work function of the metallic material was higher than that of the semiconductor material.
Unfortunately in practice, true Schottky junctions have been difficult to achieve. There is no general agreement on the reason for this, but several possible explanations have been proposed. One was given by John Bardeen in an article entitled "Surface States and Rectification at a Metal Semi-Conductor Contact," Physical Review, Vol. 71, No. 10 (May 15, 1947). In this article, Bardeen attributes the failure to achieve a true Schottky junction to the presence of interface states at the junction, with the interface states controlling the energy band bending of the semiconductor material regardless of its Fermi level. He postulated that there is a maximum number of "dangling bonds" at the junction between the two materials that could not be exceeded if a true Schottky junction was to be attained. Another theory blames the failure to achieve a true Schottky junction on quantum effects at the interface between the metallic and semiconductor materials. The problem has also been attributed to a dipole layer of charge on the surface of the semiconductor material which, being a sensitive function of surface conditions, has no unique value for a given semiconductor material (L. P. Hunter, Handbook of Semiconductor Electronics, McGraw-Hill, Inc., 1970, pages 3-4).
In practice, it has been necessary to substantially complicate the circuit fabrication process to achieve generally non-rectifying metal-semiconductor junctions. For semiconductor materials, the accepted process first requires a heavy doping of the semiconductor to move its Fermi level closer to or entirely into its conduction band. A metal or metal alloy is then deposited on the doped semiconductor and heated until the desired ohmic contact is achieved. In addition to complicating the fabrication process, the temperature cycling can make the resulting device unusable for certain applications.
A somewhat more detailed analysis of an ideal Schottky junction will be helpful in providing a background for the invention. A good reference is McKelvey, Solid State and Semiconductor Physics, Harper & Row (1965). Semiconductor materials are characterized by a valence band and a higher energy electron conduction band, separated by an energy gap known as the "forbidden zone." Each electron found in the conduction band comes from the valence band and leaves a corresponding vacancy there. This fact is used to define the "Fermi level," which is the energy level at which the probability that the number of electrons which have been excited across the energy gap at a particular temperature will be equal to the number of electrons remaining in the conduction band. For example, if the energy level density functions of the conduction and valence band are identical in magnitude and symmetrical in form about the energy gap (a condition which does not precisely occur in practice), the Fermi level will be at the exact center of the energy gap. If the valence band contains twice the density of states or energy levels as the conduction band, the Fermi level will be somewhat above the center of the energy gap to maintain the number of electrons in the conduction band equal to the number of vacancies in the valence band. If, on the other hand, the conduction band contains a higher density of states than does the valence band, the situation will be reversed and the Fermi level will lie somewhat below the center of the energy gap. In all three cases the total number of electrons in the conduction band and the vacancies in the valence band will increase with increasing temperature.
When metallic and semiconductor materials having different work functions are brought into contact with one another, a brief transient current flow takes place which transfers electrons from the material with the larger Fermi level to the one with the smaller Fermi level, thereby generating an equilibrium contact potential difference between the two. This phenomenon is illustrated in FIGS. 1(a)-(c) for the case of a contact between a metal and an n-type semiconductor crystal, in which the metal's work function, WF.sub.M, is larger than the work function WF.sub.SC associated with the semiconductor. Initially, as illustrated in FIG. 1(a), the two materials are separated by a distance d1 which is assumed to be relatively large, perhaps on the order of a centimeter. Under these circumstances, the Fermi levels of the two materials do not coincide, and the system is not in equilibrium. To achieve equilibrium, electrons would have to tunnel through the potential barrier from the semiconductor material with the higher Fermi level to the metal with the lower Fermi level until a condition of equilibrium in which the two Fermi levels coincide is attained.
As the two materials are brought closer together to a separation d2, as illustrated in FIG. 1(b), the thickness of the tunneling barrier becomes less and less, until finally the tunneling probability becomes sufficiently large that electrons penetrate the barrier, flowing from right to left in the figure. The physical separation d2 which is necessary to bring this about is a few times the interatomic distance. The flow of electrons from the semiconductor to the metal continues as the materials are brought into actual or near contact, at which point electrons tunnel freely through the barrier. An electric field arises largely within the semiconductor due to the contact potential difference. The potential energy of an electron at rest at the bottom of the conduction band in the interior of the crystal thus differs from the potential energy of such an electron at the surface by the difference in the work functions of the two materials and, as a result, the conduction and valence band edges are shifted with respect to the Fermi level as illustrated in FIG. 1(c). A positive space charge density is formed in the surface region due to the excess concentration of ionized donor atoms over the electron population, in conjunction with the electrons which tunneled through to the metal. This space charge density is just enough to produce an electric field sufficient to sustain the potential difference WF.sub.M-SC between the two materials.
In the described example, the net carrier density near the semiconductor surface is reduced from its bulk equilibrium value, and the surface layer is referred to as a depletion region. If WF.sub.M-SC is sufficiently large, the bands may be shifted with respect to the Fermi level to such an extent that the region next to the surface of the valence band is nearer the Fermi level than is the conduction band; the material immediately adjacent the surface then becomes, in effect, p-type. In this condition, the surface is then said to be inverted in conductivity type, and the surface p-type region is called an inversion region.
In the example of FIG. 1(c), a potential barrier having a height of WF.sub.M-SC is formed at the surface. The depletion region is quite thick, and there is little possibility (except under extreme circumstances) for electrons to be able to tunnel through the barrier. The contact thus forms an effective rectifier.
The formation of an ideal Schottky barrier is illustrated in FIGS. 2(a) and (b). In contrast to the rectifying junction of FIGS. 1(a)-(c), in FIGS. 2(a) and (b) the work function of the metallic material is assumed to be less than the semiconductor work function. As the materials are brought together, the semiconductor acquires a negative charge, the metallic material acquires a positive charge, and the bands shift downward at the surface rather than upward. As a result, instead of a potential barrier, an accumulation region is formed in which the electron concentration is greater than the concentration of ionized donor atoms. The excess electron concentration in the surface accumulation region gives rise to a negative space charge which supports the contact potential difference between the two materials. A non-rectifying junction in which electrons are free to flow in either direction without encountering a barrier is thus the theoretical result of the described Schottky junction.
While the above discussion has been limited to n-type semiconductors, similar effects result from p-type semiconductors. A depletion of inversion region, with an accompanying potential barrier, is formed at the interface between a p-type semiconductor and a metal whose work junction is smaller than that of the semiconductor, while an accumulation layer that gives rise to a non-rectifying jucntion is formed between the p-type semiconductor and a metal having a larger work function.
Although the ideal Schottky junction has been extensively discussed in the literature, efforts to achieve such a junction in practice have been frustrated. As mentioned above, several different theories have been proposed to explain this failure, but none of the theories have proposed a viable way to implement a solution.